Method of establishing a subscriber connection and a subscriber network

ABSTRACT

For establishing a subscriber connection in a data transmission network and to a subscriber network, a subscriber connection is established by forming a first portion of a transmission connection on the subscriber side in an electrical form and a second portion of the connection on the exchange side in an optical form by means of an optical fibre. There are optoelectric converters at both ends of the second portion of the connection. In order to get rid of transmission devices of an active subscriber network, which devices are to be positioned in the field and are difficult to maintain, a signal transmitted by a transmission device of a subscriber is connected, without correcting the attenuation and distortion caused by the first portion, via one of the optoelectric converters, to said said portion correspondingly, a signal is transmitted from the exchange towards the subscriber, which signal is intended for the first portion and is at first connected via the other one of the optoelectric converters to the said portion.

BACKGROUND OF THE INVENTION

The invention relates to a method of establishing a subscriberconnection and to a subscriber network. In the method a subscriberconnection is established by forming a first portion of a transmissionconnection on the subscriber side in an electrical form and a secondportion of the connection on the exchange side in an optical form bymeans of an optical fibre, whereby there are optoelectric converters atboth ends of said second portion of the connection. A subscriber networkaccording to the invention is useful for practicing the method.

Optical fibre is a self-evident choice for a transmission medium of atrunk network, because trunk connections generally require a largetransmission capacity, transmission distances used are long and cableroutes ready for use are often available. On the other hand, thesituation in a subscriber connection is often quite the opposite andsufficient reasons for using optical fibre rarely exist.

Naturally, with a continuously increasing need for speed, the situationis changing in a more advantageous direction for the optical fibre, butno significant savings can be expected in the total costs, mainlyresulting from cable installation costs. A wish is, however, to fit outthe subscriber network as well with as much optical fibre as possible,because it is obvious that it will be required in the future. Costs ofreconstructing a subscriber network are very high and, as to the time,it is a matter of decades in this connection. Consequently, it would beworth while being prepared for coming needs and providing facilities forthe future at present already.

Accordingly, teleoperators try to increase the number of fibres in asubscriber network, as far as it is economically possible. In practicethere are two possibilities of increasing the number of fibres. Firstly,optical fibre can be installed at relatively low additional costs inconnection with cable supplements. However, the fibre often remainsawaiting future users. The other way is to find customers who need highspeed and thus optical fibre and who are ready to pay for it.

The fact is that high costs are the worst obstacle to introducing thefibre into the subscriber network.

In a so-called active subscriber network, there are transmission devicesalso "in the field" (in the area between a terminal exchange and asubscriber). The object of an active subscriber network is to divide thecosts of constructing a transmission connection among severalsubscribers and to utilize existing copper cabling as far as possible.FIG. 1 shows the principle of an active subscriber network. Atransmission equipment, joining an optical portion and a copper portionof a subscriber connection and having a box indicated by reference markB, is positioned in the field in such a place where subscribers 12 arereached by means of an existing copper cabling 13. The transmissionequipment typically comprises a multiplexer 11 and baseband modems 14for each subscriber connection. Correspondingly, each subscriber has amodem 14 of its own, which modems can operate at different rates. Aconnection from the multiplexer 11 to an exchange 16 is formed by meansof an optical fibre 15. The maximum length of the copper portion 13 istypically 0.5 . . . 1 km and the length of the optical portion 15typically 1 . . . 5 km.

By means of the arrangement described above, the costs of establishingan optical fibre cable connection can be divided among severalsubscribers 12. The costs are further lowered by possible finishedpipings, occurring most probably just in the vicinity of exchanges.Consequently, the most expensive part of the connections, i.e.individual connections to subscribers, would consist of existing coppercable connections, which means that they would be practically almostfree of cost.

On the basis of crosstalk between the pairs of a pair cable and anattenuation of a copper cable, it can be estimated that, by using onequad, it is possible in practice to achieve a distance of one kilometerat the rate of 8 Mbit/s and a distance of 2,5 km at the rate of 2Mbit/s. By using 2 or 3 quads and by accepting a distance of about 500meters, the rate can be even 34 Mbit/s.

By means of the active subscriber network described above, connectionsfaster than 2 Mbit/s are thus achieved at reasonable costs. If anoptical cable is terminated at a distance of about one kilometer fromthe subscribers at the maximum, a conventional G.703/2 Mbit/s interfaceand a transmission according to that can be used as such (no separatemodem is necessary).

However, an active subscriber network has drawbacks, too, which will bedescribed in the following. Firstly, the large size of the devices leadsto the fact that it may be difficult to find a place in the field wherethe devices required can be positioned. Additionally, the structure ofthe devices ought to be modular so that interfaces of different kindscan be provided. Simultaneously, the structure ought to withstand hardenvironmental conditions. Moreover, the devices consume so much powerthat it is not possible in practice to feed such a quantity of power allthe way from the exchange.

Control over the devices constitutes a further problem. There is onemore place in a transmission connection of an active subscriber networkinto which a modem required shall be introduced. It is necessary to knowhow to configurate this modem right or how to arrange a possibility ofremote configuration. This concerns multiplexers, too. Additionally, itshall be possible to control the operation of the devices, and e.g.changes in configuration for one subscriber must not cause any harm tothe other subscribers. It is also very laborious to maintain and replacetransmission devices in the field.

SUMMARY OF THE INVENTION

The object of the present invention is thus to avoid the above drawbacksand to provide a method by which fast subscriber connections (≧2 Mbit/s)can be established in an economically profitable way. This object isachieved by means of the method of the invention, which is characterizedin that a signal transmitted by a transmission device of a subscriber isconnected, without correcting the attenuation and distortion caused bysaid first portion, via an optoelectric converter to said second portionand that, correspondingly, a signal is transmitted from the exchangetowards the subscriber, which signal is intended for said first portionand is at first connected via the optoelectric converter to said secondportion. The subscriber network of the invention is, in turn, useful forpracticing the method.

The basic idea of the invention is to "stretch" a copper cable startingfrom a subscriber towards the exchange by means of an optical link insuch a way that, seen from the end, the whole transmission connectioncomprising both the electrical portion (copper portion) and the opticalportion looks like a copper cable, the length of which is identical withthe length of the copper portion, as far as distortion and attenuationare concerned. The subscriber connection can thus be made physicallyconsiderably longer than it would be otherwise, i.e. due to distortion,attenuation and crosstalk.

Due to the solution provided by the invention, the modems required canalways be positioned in exchange premises and the structure of thedevice to be positioned in the field is simple and always similar. Thepower required by the device to be positioned in the field is reasonableand permits remote power supply from the exchange. The device does notneed to know either what is transmitted by the connections, but ittransmits a signal occurring in a cable as such, and therefore, noconfiguration is needed. Additionally, the solution of the inventionmakes the maintenance and replacement of the equipment simpler.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described more accuratelyreferring to the examples according to FIGS. 2 to 4 of the attacheddrawings, in which:

FIG. 1 shows the principle of an active subscriber network,

FIG. 2 shows an establishment of a subscriber connection by using theprinciple of the invention,

FIG. 3 shows a solution of a preferred embodiment of the invention, inwhich one optical link transmits data of more than one channel,

FIG. 4 shows a typical application of the solution of the invention,

FIG. 5 shows a second application of the solution of the invention.

DETAILED DESCRIPTION

FIG. 2 shows the principle of one channel of a subscriber connectionestablished by the method of the invention. An optoelectric converter 20is positioned at the exchange 16 end of a copper cabling 13 startingfrom a subscriber 12, e.g. a corporation, preferably in a placecorresponding to the transmission equipment of FIG. 1, in whichsubscribers are reached by means of the existing copper cabling 13. Thisconverter converts an analog electric signal coming from the subscriberinto an optical form for an optical portion 15. At the opposite end ofthe optical portion (in the input of the exchange 16), there is anotheroptoelectric converter 20, which converts the optical signal back to theanalog electric form, in which form it is connected to the exchange in amanner known per se. (The term optoelectric converter is used in thisconnection irrespective of which conversion direction (transmissiondirection) is in question in each case.) The optical portion 15 and theconverters 20 at the ends thereof thus form an optical link, by whichthe copper cabling 13 of each subscriber connection is stretched longer,as far as to the exchange 16 in practice. A signal transmitted by atransmission device 14 of the subscriber is then connected as such, i.e.distorted and attenuated by the copper cabling portion, to the opticalportion 15, and these distortions and attenuations of the subscriberconnection are not corrected until at the exchange interface. In arespective way, a signal is transmitted from the exchange in anothertransmission direction, which signal is intended for the copper cablingportion. This signal is at first connected to the optical portion bymeans of an optoelectric electric converter and then converted back tothe electrical form and connected directly to the copper cabling portion13. In this transmission direction, attenuations and distortions do notoccur until at the final end of the transmission connection, i.e. at thecopper cabling portion. The whole transmission connection can thus bemade physically considerably longer than it would be otherwise, i.e. dueto distortion and attenuation, because practically only the copperportion causes distortion and attenuation in a transmission connectionestablished in this way.

In practice, as a line code can serve in both transmission directionse.g. a 2B1Q code known per se, but any other line code by which thespectrum of a signal can be made suitable for a transmission path isequally possible. (The line code represents how bits are changed into ananalog form for a transmission connection.)

FIG. 3 shows a preferred practical embodiment of the invention. In thiscase an analog signal to be obtained from the copper cabling portion 13is digitized by analog/digital converters 34 for the time of atransmission performed by the optical portion 15, and one optical linktransmits data of more than one channel (subscriber). The copper cabling13 of more than one subscriber (not shown in FIG. 3) is thus connectedvia a corresponding isolation transformer 31, a low-pass filter 32, again control circuit 33 (by which the signal is scaled to a suitablelevel) and an A/D converter 34 mentioned above to a common multiplexer35, the output of which is connected to the optical portion (fibre) 15via an optoelectric converter 20a. The converter 20a comprises in thiscase an optical transmitter, e.g. a laser or a LED. The signal comingfrom the optical portion (fibre) 15 is received by another optoelectricconverter 20b, which comprises a receiver, e.g. a PIN diode, for theoptical signal. The output of the converter 20b is connected to ademultiplexer 38, which delivers the channels to separate D/A converters39, by which the signals are converted back to the analog form andconnected in a manner known per se via respective low-pass filters 32and isolation transformers 31 to the exchange equipment.

FIG. 3 shows the implementation only in one transmission direction (froma subscriber to an exchange). The other transmission direction has beenimplemented in a corresponding manner. It shall be noted that in spiteof the fact that the signals are digitized in this example for the timeof a transmission performed by the optical link, the signals at the endsof the optical link are still analog. The optical portion 15 is thusconnected by means of an electric coupling in an analog form directly tothe copper cabling portion, on the one hand, and to the exchangeinterface, on the other hand. A multiplexer unit to be positioned in thefield is indicated by reference mark H1 in FIG. 3.

Instead of digitizing a signal temporarily for the time of atransmission in the manner described above, a purely analog solution canbe used, in which the separate channels are modulated to differentfrequencies to the same fibre.

FIG. 4 shows a typical application of the solution of the invention. Amultiplexer/demultiplexer unit H, comprising the optoelectric convertersdescribed above and being (in both transmission directions) identicalwith the unit shown in FIG. 3, for instance, is positioned at a terminalexchange PK, and another similar unit H is positioned in the mannerdescribed above in the field in a place where the subscribers 12 can bereached by means of the existing copper cabling 13. The power supply ofthe multiplexer unit H in the field occurs from the terminal exchangePK. The modems 14 are situated in their known places, i.e. in theexchange and with the subscribers (cf. FIG. 1). If the copper cablingportion is short enough, the G.703 interface will do as such and noseparate modems are needed at all.

Another application of the solution of the invention is shown in FIG. 5.Because there are many terminal exchanges PK, it may not always bepreferable e.g. for labour and network management to position alldifferent data transmission devices in every place. Accordingly, datatransmission devices and staff can be concentrated by positioning themultiplexer unit H at the subscriber end of the optical portion 15 in aterminal exchange PK and the multiplexer unit at the opposite end of theoptical portion in a tandem exchange SK. Consequently, the opticalportion 15 is in this case between the tandem exchange and the terminalexchange and the copper portion 13 between the terminal exchange and thesubscriber.

Though the invention has been described above referring to the examplesof the attached drawings, it is clear that the invention is notrestricted to it, but it can be modified in many ways within the scopeof the inventive idea presented above and in the attached claims. Thetransmission apparatuses of the subscriber, for instance, can be ofdifferent types.

I claim:
 1. A method for establishing a two-way communicationsconnection between a subscriber and an exchange in a data transmissionnetwork, comprising the steps of:(a) providing a first portion of theconnection, on a subscriber side, in an electrical form, between thesubscriber, and a second portion which is closer to the exchange than issaid first portion; (b) providing said second portion, on an exchangeside, in an optical form, along an optical fiber, said second portionhaving one end nearest said first portion, and an opposite end nearestsaid exchange, along said optical fiber; (c) providing respectiveoptoelectric converters at said one and opposite ends of said secondportion, for converting optical signals to electrical signals, and viceversa; (d) providing a remainder of the connection between the one ofsaid optoelectric converters provided at said opposite end of saidsecond portion, and said exchange; (e) connecting a signal transmittedby a transmission device of said subscriber, along a communications pathextending from said subscriber via said first portion, said optoelectricconverter at said one end, said second portion, said optoelectricconverter at said opposite end, and said remainder, withoutsubstantially correcting attenuation and distortion caused by said firstportion between said first portion and said opposite end of said secondportion, this signal having an analog form between the subscriber andsaid optoelectric converter at said one end, (f) correcting saidattenuation and distortion, at a location along said path between saidoptoelectric converter at said opposite end, and said exchange; and (g)transmitting a signal in analog form along said path from the exchangetowards said subscriber, this signal having an analog form between theexchange and said said optoelectric converter at said opposite end.
 2. Amethod for establishing a two-way communications connection between eachof a plurality of subscribers and an exchange in a data transmissionnetwork, comprising the steps of:(a) providing a first portion of theconnection, on a subscriber side, in an electrical form, between eachsubscriber, and a second portion which is closer to the exchange than iseach said first portion; (b) providing said second portion, on anexchange side, in an optical form, along an optical fiber, said secondportion having one end nearest each said first portion, and an oppositeend nearest said exchange, along said optical fiber; (c) providingrespective optoelectric converters at said one and opposite ends of saidsecond portion, for converting optical signals to electrical signals,and vice versa; (d) providing a remainder of the connection between theone of said optoelectric converters provided at said opposite end ofsaid second portion, and said exchange; (e) connecting a signaltransmitted by a respective transmission device of each of a pluralityof said subscribers, along a communications path extending from therespective said subscriber via the respective said first portions, saidoptoelectric converter at said one end, said second portion, saidoptoelectric converter at said opposite end, and said remainder, withoutsubstantially correcting attenuation and distortion caused by therespective said first portions between said first portion and saidopposite end of said second portion, this signal having an analog formbetween the respective subscribers and said optoelectric converter atsaid one end, (f) correcting said attenuation and distortion, at alocation along said path between said optoelectric converter at saidopposite end, and said exchange; and (g) transmitting on a respectiveplurality of channels respective signals in analog form along said pathfrom the exchange towards the respective said subscribers, these signalshaving an analog form between the exchange and said said optoelectricconverter at said opposite end.
 3. The method of claim 2, furtherincluding:temporarily digitizing said signals for transmission alongsaid optical fiber, at respective locations between said first portionsand said remainder.
 4. A data transmission network for providing atwo-way communications connection between an exchange and at least onesubscriber, comprising:(a) for each said subscriber, a respective firstportion of the connection, on a subscriber side, arranged for signallingelectrically, between the subscriber, and a second portion which iscloser to the exchange than is each said first portion; (b) a secondportion, on an exchange side, arranged for signalling optically, alongan optical fiber, said second portion having one end nearest each saidfirst portion, and an opposite end nearest said exchange, along saidoptical fiber; (c) respective optoelectric converters provided at saidone and opposite ends of said second portion, for converting opticalsignals to electrical signals, and vice versa; (d) a remainder of theconnection, between the one of said optoelectric converters provided atsaid opposite end of said second portion, and said exchange; (e) foreach subscriber, a transmission device for connecting a respectivesignal, along a respective communications path extending from therespective said subscriber via the respective said first portion, saidoptoelectric converter at said one end, said second portion, saidoptoelectric converter at said opposite end, and said remainder, withoutsubstantially correcting attenuation and distortion caused by said firstportion between said first portion and said opposite end of said secondportion, this signal having an analog form between the subscriber andsaid optoelectric converter at said one end, (f) means for correctingsaid attenuation and distortion, at a location along said path betweensaid optoelectric converter at said opposite end, and said exchange; and(g) means for transmitting a respective signal in analog form along eachsaid path from the exchange towards a respective said subscriber, eachsuch signal having an analog form between the exchange and said saidoptoelectric converter at said opposite end.